The present invention relates to systems for measuring rotor shaft position for motor control purposes and the like and more particularly relates to phase analog resolver to digital converters which provide an indication of rotor position using a high speed digital counter to measure the phase shift between stator and rotor signals.
Resolvers ordinarily comprise rotating transformers which are constructed and arranged to produce different voltages as they turn. Resolvers consist of two components, a stator which is stationary and a rotor which is fixed to a rotating shaft having an angular position to be measured. Resolvers also have three electrical windings two of which are located on the stator (and are ordinarily offset by 90.degree. electrical from one another) and one of which is installed on the rotor. As the shaft turns and the rotor rotates with a respect to the stator, the rotor winding changes alignment with the stator windings so that the rotor winding is in phase with different stator windings at different points during its rotation.
When the stator windings are excited with sinewave and cosinewave type reference signals a voltage is induced in the rotor winding as the rotor rotates which is a function of the reference signals applied to the stator windings and the rotor position. As a consequence the rotor position can be determined by measuring the phase shift or lag of the rotor voltage with respect to one of the reference voltages (either the sinewave type signal or cosinewave type signal) supplied to the electrical windings on the stator.
Phase analog resolver to digital converters are commonly used to measure the phase shift or lag between the stator reference and rotor signals. Phase analog detectors use high speed digital counters to measure the phase lag between the stator and rotor voltages with the counter being enabled to count the pulses in a high speed clock signal when the voltage applied to the stator winding crosses zero with a positive slope and being stopped when the rotor winding voltage crosses zero with a positive slope. The resulting counter value is stored in a latch for transfer to a microprocessor system as the counter is reset to prepare for the next cycle.
Unfortunately, while phase analog resolver to digital converters provide a simple method of generating a digital representation of mechanical shaft position, there are important weaknesses in the phase analog detection technique. When the zero crossing points for the reference and rotor signals occur at almost the same time (i.e. in "near coincidence") the counter can give incorrect position indications since the circuitry may not have sufficient time to clear previous data data from the system components. Additionally, the circuitry can skip rotor position samples or readings when the zero crossing point of the rotor signal changes from just after the zero crossing point of the stator reference signal to just before the zero crossing point of the stator reference signal (i.e. two rotor zero crossing points occur before a stator reference signal zero crossing point) as it automatically does once for every revolution of the rotor. Erroneous position readings resulting from near coincidence of the zero crossing points and skipped rotor position readings are serious problems that frequently cause major difficulties for controllers which use the feedback information provided by phase analog resolver to digital converters.
It is therefore an object of the present invention to provide a phase analog type resolver to digital converter which produces rotor position measurements which are not subject to error due to near coincidence between the zero crossing points of the stator reference and the rotor signals or skipped rotor position readings.
It is a further object of the present invention to provide a phase analog type resolver to digital converter which produces a more accurate measurement of rotor position at the same time that it avoids problems due to skipped rotor position readings and incorrect position indications due to failure of its circuitry to clear previous data.
It is yet another object of the present invention to provide a resolver to digital converter which functions to provide rotor position information more reliably and accurately at all times and under all conditions for use by motor controllers and the like.